The metal values in ores usually are present in oxide, carbonate and/or hydroxide form in intimate admixture with gangue constituents which also are metal oxides. "Gangue" is considered to be the minerals and rock material mixed with the metallic ore which are themselves valueless. The precise chemical form of the gangue constituents depends on the ore but usually includes silica, alumina, magnesia and lime.
Since the initial ore or concentrate thereof contains an intimate mixture of metals in oxide, carbonate and/or hydroxide forms of very similar properties, the desired metal for which the ore was originally mined cannot readily be separated in its oxide, carbonate, and/or hydroxide form from the gangue material.
The recovery of metals from their ores generally has involved a smelting procedure using coke and a flux. The liquid metal is tapped at intervals from the smelter, followed by solidification, cleaning and sizing of the metal or alloy. The process is polluting in nature, difficult to control, is subject to heavy product losses in slag, hence requiring recycling, and impure products often are formed.